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WAVE CYCLONES

Wave cyclones form along fronts. Since our most familiar front is the polar front we will use thisone for our example. Here, the polar easterlies are blowing from east to west along the northern

side of the front, and the prevailing westerlies are blowing from west to east along the southern

side. At the front, waves will be formed, just exactly like waves are formed in the ocean.

cean waves are formed at the interface of two fluids of different densities !! air and water.

Atmospheric waves such as we are now talking about are also formed at such an interface. "nthis case it is also the interface between two fluids of different densities, being colder, denser air

from the north and warmer, less dense air from the south.

As these winds blow past each other, ripples will form. #hey will form in ripples of differentsi$es and lengths, and when the combinations are right, they will build from ripples into waves.

%et&s take a look at one wave as it develops.

Starting with a straight!line polar front, under the right conditions a small ripple or wave willbegin to form. ne it has begun, it will build in much the same fashion as an ocean wave does.

#he warm m' air (westerly wind) from the south will now impinge on the south side of theripple, while the cold c' air from the north (easterly wind) will impinge on the north side.

#his will build the wave deeper and deeper until at some point two things will happen. #heconverging air will begin to revolve and lift, creating a local low pressure area, and as that

happens the low will become enclosed in roughly circular isobars, with a warm front at the head

of the m' air and a cold front behind it, where the leading edge of the following c' air is. #hese

two fronts originate at the apex of what was the original wave and fan out to the south.

#his newly formed depression is what we know of as our synoptic scale weather maker. nceformed it will track generally in the direction of the isobars in the warm sector, which willgenerally be in the same direction as the (polar) front along which it was formed. #his you will

note is also the direction of the upper air winds blowing along that front, namely the jet stream.

#he cold front tends to move faster than the warm front. #his means that as the depression

moves along, the cold front overtakes the warm front, first at the top near the center and

gradually farther and farther down. #his is the occlusion of the system, and the new front thus

formed is the occluded front we have discussed. #he net result is the gradual uplifting of thewarm sector of the depression to the point when the entire warm sector is lifted off the ground.

nce this happens the surface pressure gradient, and therefore the driving force for the system, is

lost and the low is said to fill and eventually disintegrate altogether.

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WEATHER ASSOCIATED WITH FRONTAL PASSAGE

#he weather associated with our model frontal system provides a good way to describe theweather building mechanics of the fronts. f course this is an ideali$ed system and nature rarely

follows such ideal patterns.

1igure +!- shows these cloud and precipitation patterns. 1igure +!+ (in #he Atmosphere)

shows a satellite image of a nearly ideal system. 2emembering that the winds aloft are generally

westerly, and it is the winds aloft that generally guide the surface depressions, we can accept thatthese systems will move from east to west, more or less. Sometimes these systems can move in a

more northerly direction, especially east of the 3ississippi. #he systems move at a rate of about

4+ to 5+ km6hr (+ to 45 kts) and so cover between 4/+ to 7++ nm per day.

"f you are an observer at point A, the system will move over you so that you will end up (relative

to the system) at point 8. #he first signs you will see are the high, wispy mare&s tails, or 9i.

#hese may be +++ km (over 5++ nm) ahead of the warm front. #he barometer will begin to fall.

How fast it is falling may indicate the speed of approach or the intensity of the low. ver thenext 4 to 4/ hours, the clouds will thicken and lower, from 9i to 9s to As to S and finally the

precipitation will begin with the :s clouds.

As the front arrives, precipitation increases, and as it passes the temperature will rise, the

barometer will stop falling, and the wind will shift from S8 to SW. (:ote that this is a veeringwind. (A shift counter!clockwise is called a backing wind.)#he precipitation will stop, the skies

will become mostly clear, although Ac or 9u may well be present due to the moisture content of

the warm air. #hese conditions will remain mostly steady until the near approach of the cold

front, a day or two later. nce you see the 9b clouds identifying the cold front it will be uponyou shortly, within a couple of hours. #his brings the more vigorous and sometimes violent

weather of the cold front. 9b clouds, heavy downpours, thunderstorms, possibly hail, tornadoes,

and microbursts come with the abrupt passage of the cold front.

Sometimes a line of s;uall!line thunderstorms precedes the front by a short distance, but these

can be violent too, so they are not a warning of any use to us. #he best warning is to know it willbe coming, watch the western sky, watch the barometer for the beginning of a rise, watch the

thermometer for the beginning of a rise, and watch the winds for the beginning of a shift from

the SW to the :W.

When the cold front passes all of the above occur and there will be a period of cold, clear high

pressure for a day or three before the next system comes along.

An observer located at position 1, however, will observe a very different set of conditions as the

center of the low passes south of them. #his observer will pass through the occluded portion of

the storm, and furthermore their winds will be out of the north!!backing around from the 8 to the:8 to the :W. All this wind will be drawing down cold air from the north into this observer&s

part of the system, so this area will more likely get snow, while the observer to the south is

getting rain or sleet.

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#here will be no sharp contrasts at the fronts such as we saw farther south, but rather a general

drop in temperature and pressure as the occluded front approaches, a general and solid overcast

area with continuous precipitation.

#he occluded front moves slower than the others, so it tends to drag behind, thus